1. Field of the Invention
The present invention relates to a centrifugal fan, and more particularly, to a centrifugal fan which can reduce noise at air blowing.
2. Description of the Related Art
A centrifugal fan is configured by providing an impeller in a scroll casing. The impeller has a plurality of blades disposed around a rotation shaft of a motor, and the scroll casing has a suction opening and a discharge opening. Air suctioned from the suction opening flows from the center of the impeller into between the blades, and is discharged outward in the radial direction of the impeller by a fluid force due to a centrifugal action from the rotation of the impeller. The air discharged from the outer circumference of the impeller passes through the scroll casing to become high-pressure air, and is discharged from the discharge opening.
This centrifugal fan is widely used for cooling, ventilation, and air conditioning in home appliances, OA devices, and industrial equipment, an air blower for a vehicle, and the like. However, due to the configuration of the centrifugal fan, the air blowing performance and noise are significantly influenced by the blade shape of the impeller and the shape of the scroll casing.
Therefore, there has been proposed a centrifugal fan which optimizes a blade shape of an impeller to reduce noise (see, for example, JP-A-S63-289295).
FIG. 8 is a plan view illustrating a centrifugal fan described in JP-A-S63-289295, and FIG. 9 is a perspective view illustrating an impeller of FIG. 8. The impeller 120 of the centrifugal fan includes a plurality of blades 123 installed between a main plate 121 and a sub plate 122, the outer circumferential side of the blades 123 rotates with delay from the inner circumferential side of the blades 123 in the rotation direction of the impeller 120. A scroll casing 127 is attached to the impeller 120 to blow air.
The blown air is suctioned from the suction opening 140 of the impeller 120, discharged from the outer circumference of the impeller 120 by the fluid force due to the centrifugal action of the blades 123 of the impeller 120, guided to a discharge opening 141 of the scroll casing 127 along the scroll casing 127 surrounding the outer circumference of the impeller 120, and discharged outside. In this blade configuration in which the outer circumferential side of the blades 123 rotates with delay from the inner circumferential side of the blades 123 in the rotation direction of the impeller 120, the blades are backward inclined blades and have a curved blade shape inclined backward in the rotation direction. The centrifugal fan having that blade shape is generally called a turbofan.
In the turbofan shown in FIG. 8, the plurality of blades 123 are interposed between the main plate 121 and the sub plate 122 having the same outside diameter, and the blades have trailing edges cut such that a blade arc 135 on the main plate side is shorter than a blade arc 134 on the sub plate side. Therefore, a time difference is generated between a time when each trailing edge 131 located on the main plate side crosses a tongue part 129 of the casing and a time when a corresponding trailing edge 132 located on the sub plate side crosses the tongue part 129 of the casing, such that pressure fluctuation occurring when the blades 123 cross the tongue part 129 of the casing is dispersed temporally, and sound generating energy is dispersed, so that generation of noise can be suppressed.
The turbofan described in JP-A-S63-289295 realizes suppression of noise when air is blown, by the shape of the blades 123. However, when air discharged from the outer circumference of the impeller collides with an inner wall surface of the scroll casing 127, a portion of the air flows back to the suction opening 140 so as to interfere with air suctioned into the suction opening 140, so that disturbance occurs in the air flow at the suction opening 140, and the disturbance of the air flow causes noise. Moreover, the disturbance of the air flow reduces an air flow in the suction opening 140, and thus the air blowing performance is deteriorated.